Provided by: liblttng-ust-dev_2.13.6-2ubuntu1_amd64 bug

NAME

       lttng_ust_tracelog, lttng_ust_vtracelog - LTTng-UST printf(3)-like interface with a log
       level

SYNOPSIS

       #include <lttng/tracelog.h>

       #define lttng_ust_tracelog(level, fmt, ...)
       #define lttng_ust_vtracelog(level, fmt, ap)

       Link with:

       •   -llttng-ust

       •   If you define _LGPL_SOURCE before including <lttng/tracelog.h> (directly or
           indirectly): -llttng-ust-common

DESCRIPTION

       The LTTng-UST lttng_ust_tracelog() and lttng_ust_vtracelog() API allows you to trace your
       application with the help of simple printf(3)-like and vprintf(3)-like macros, with an
       additional parameter for the desired log level.

       The fmt argument is passed directly as the fmt parameter of vasprintf(3), as well as:

       For lttng_ust_tracelog()
           The optional parameters following fmt.

       For lttng_ust_vtracelog()
           The ap parameter as the ap parameter of vasprintf(3) (va_list type).

       The purpose of lttng_ust_tracelog() and lttng_ust_vtracelog() is to ease the migration
       from logging to tracing.

       The available values for the level parameter are:

       LTTNG_UST_TRACEPOINT_LOGLEVEL_EMERG
           System is unusable.

       LTTNG_UST_TRACEPOINT_LOGLEVEL_ALERT
           Action must be taken immediately.

       LTTNG_UST_TRACEPOINT_LOGLEVEL_CRIT
           Critical conditions.

       LTTNG_UST_TRACEPOINT_LOGLEVEL_ERR
           Error conditions.

       LTTNG_UST_TRACEPOINT_LOGLEVEL_WARNING
           Warning conditions.

       LTTNG_UST_TRACEPOINT_LOGLEVEL_NOTICE
           Normal, but significant, condition.

       LTTNG_UST_TRACEPOINT_LOGLEVEL_INFO
           Informational message.

       LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_SYSTEM
           Debug information with system-level scope (set of programs).

       LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_PROGRAM
           Debug information with program-level scope (set of processes).

       LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_PROCESS
           Debug information with process-level scope (set of modules).

       LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_MODULE
           Debug information with module (executable/library) scope (set of units).

       LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_UNIT
           Debug information with compilation unit scope (set of functions).

       LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_FUNCTION
           Debug information with function-level scope.

       LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG_LINE
           Debug information with line-level scope (default log level).

       LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG
           Debug-level message.

       To use lttng_ust_tracelog() or lttng_ust_vtracelog(), include <lttng/tracelog.h> where you
       need it, and link your application with liblttng-ust and liblttng-ust-common. See the
       EXAMPLE section below for a complete usage example.

       Once your application is instrumented with lttng_ust_tracelog() and/or
       lttng_ust_vtracelog() calls and ready to run, use lttng-enable-event(1) to enable the
       lttng_ust_tracelog:* event. You can isolate specific log levels with the --loglevel and
       --loglevel-only options of this command.

       The lttng_ust_tracelog() and lttng_ust_vtracelog() events contain the following fields:

       ┌───────────┬──────────────────────────────────┐
       │Field nameDescription                      │
       ├───────────┼──────────────────────────────────┤
       │           │                                  │
       │line       │ Line in source file where        │
       │           │ lttng_ust_tracelog() was called. │
       ├───────────┼──────────────────────────────────┤
       │           │                                  │
       │file       │ Source file from which           │
       │           │ lttng_ust_tracelog() was called. │
       ├───────────┼──────────────────────────────────┤
       │           │                                  │
       │func       │ Function name from which         │
       │           │ lttng_ust_tracelog() was called. │
       ├───────────┼──────────────────────────────────┤
       │           │                                  │
       │msg        │ Formatted string output.         │
       └───────────┴──────────────────────────────────┘

       If you do not need to attach a specific log level to a
       lttng_ust_tracelog()/lttng_ust_vtracelog() call, use lttng_ust_tracef(3) instead.

       See also the LIMITATIONS section below for important limitations to consider when using
       lttng_ust_tracelog() or lttng_ust_vtracelog().

EXAMPLE

       Here’s a usage example of lttng_ust_tracelog():

           #include <stdlib.h>
           #include <lttng/tracelog.h>

           int main(int argc, char *argv[])
           {
               int i;

               if (argc < 2) {
                   lttng_ust_tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_CRIT,
                                      "Not enough arguments: %d", argc);
                   return EXIT_FAILURE;
               }

               lttng_ust_tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_INFO,
                                  "Starting app with %d arguments", argc);

               for (i = 0; i < argc; i++) {
                   lttng_ust_tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_DEBUG,
                                      "Argument %d: %s", i, argv[i]);
               }

               lttng_ust_tracelog(LTTNG_UST_TRACEPOINT_LOGLEVEL_INFO,
                                  "Exiting app");
               return EXIT_SUCCESS;
           }

       This C source file, saved as app.c, can be compiled into a program like this:

           $ cc -o app app.c -llttng-ust -llttng-ust-common

       You can create an LTTng tracing session, enable all the lttng_ust_tracelog() events, and
       start the created tracing session like this:

           $ lttng create my-session
           $ lttng enable-event --userspace 'lttng_ust_tracelog:*'
           $ lttng start

       Or you can enable lttng_ust_tracelog() events matching a log level at least as severe as a
       given log level:

           $ lttng enable-event --userspace 'lttng_ust_tracelog:*' \
                                --loglevel=INFO

       Next, start the program to be traced:

           $ ./app a few arguments passed to this application

       Finally, stop the tracing session, and inspect the recorded events:

           $ lttng stop
           $ lttng view

LIMITATIONS

       The lttng_ust_tracelog() and lttng_ust_vtracelog() utility macros were developed to make
       user space tracing super simple, albeit with notable disadvantages compared to custom,
       full-fledged tracepoint providers:

       •   All generated events have the same provider/event names.

       •   There’s no static type checking.

       •   The only event field with user data you actually get, named msg, is a string
           potentially containing the values you passed to the macro using your own format. This
           also means that you cannot use filtering using a custom expression at run time because
           there are no isolated fields.

       •   Since lttng_ust_tracelog() and lttng_ust_vtracelog() use C standard library’s
           vasprintf(3) function in the background to format the strings at run time, their
           expected performance is lower than using custom tracepoint providers with typed
           fields, which do not require a conversion to a string.

       •   Generally, a string containing the textual representation of the user data fields is
           not as compact as binary fields in the resulting trace.

       Thus, lttng_ust_tracelog()/lttng_ust_vtracelog() are useful for quick prototyping and
       debugging, but should not be considered for any permanent/serious application
       instrumentation.

       lttng_ust_vtracelog() does not have a STAP_PROBEV() call, because STAP_PROBEV() does not
       support va_list. If you need it, you should emit this call yourself.

       See lttng-ust(3) to learn more about custom tracepoint providers.

BUGS

       If you encounter any issue or usability problem, please report it on the LTTng bug tracker
       <https://bugs.lttng.org/projects/lttng-ust>.

RESOURCES

       •   LTTng project website <http://lttng.org>

       •   LTTng documentation <http://lttng.org/docs>

       •   Git repositories <http://git.lttng.org>

       •   GitHub organization <http://github.com/lttng>

       •   Continuous integration <http://ci.lttng.org/>

       •   Mailing list <http://lists.lttng.org> for support and development: lttng-
           dev@lists.lttng.org

       •   IRC channel <irc://irc.oftc.net/lttng>: #lttng on irc.oftc.net

COPYRIGHTS

       This macro is part of the LTTng-UST project.

       This macro is distributed under the GNU Lesser General Public License, version 2.1
       <http://www.gnu.org/licenses/old-licenses/lgpl-2.1.en.html>. See the for more details.

THANKS

       Thanks to Ericsson for funding this work, providing real-life use cases, and testing.

       Special thanks to Michel Dagenais and the DORSAL laboratory
       <http://www.dorsal.polymtl.ca/> at École Polytechnique de Montréal for the LTTng journey.

AUTHORS

       LTTng-UST was originally written by Mathieu Desnoyers, with additional contributions from
       various other people. It is currently maintained by Mathieu Desnoyers
       <mailto:mathieu.desnoyers@efficios.com>.

SEE ALSO

       lttng_ust_tracef(3), lttng_ust_vtracef(3), lttng-ust(3), lttng(1), printf(3)